Profiling the Phospho-status of the BKCa Channel α Subunit in Rat Brain Reveals Unexpected Patterns and Complexity

Molecular diversity of ion channel structure and function underlies variability in electrical signaling in nerve, muscle, and non-excitable cells. Protein phosphorylation and alternative splicing of pre-mRNA are two important mechanisms to generate structural and functional diversity of ion channels. However, systematic mass spectrometric analyses of in vivo phosphorylation and splice variants of ion channels in native tissues are largely lacking. Mammalian large-conductance calcium-activated potassium (BKCa) channels are tetramers of alpha subunits (BK alpha) either alone or together with beta subunits, exhibit exceptionally large single channel conductance, and are dually activated by membrane depolarization and intracellular Ca2+. The cytoplasmic C terminus of BK alpha is subjected to extensive pre-mRNA splicing and, as predicted by several algorithms, offers numerous phospho-acceptor amino acids. Here we use nanoflow liquid chromatography tandem mass spectrometry on BKCa channels affinity-purified from rat brain to analyze in vivo BK alpha phosphorylation and splicing. We found 7 splice variations and identified as many as 30 Ser/Thr in vivo phosphorylation sites; most of which were not predicted by commonly used algorithms. Of the identified phosphosites 23 are located in the C terminus, four were found on splice insertions. Electrophysiological analyses of phosphoand dephosphomimetic mutants transiently expressed in HEK-293 cells suggest that phosphorylation of BK alpha differentially modulates the voltage- and Ca2+-dependence of channel activation. These results demonstrate that the pore-forming subunit of BKCa channels is extensively phosphorylated in the mammalian brain providing a molecular basis for the regulation of firing pattern and excitability through dynamic modification of BK alpha structure and function. Molecular & Cellular Proteomics 7: 2188-2198, 2008.